Nutrient enrichment experiments are a potent and well-established tool for understanding nutrient uptake and transport dynamics in streams. Most applications of this tool involve nutrient additions which produce a new, slightly enriched steady state in a stream reach from which empirical determination of downstream nutrient declines can be used to estimate uptake parameters. While this traditional steady state nutrient enrichment approach is easy to apply, it is limited in its ability to address important hydrologic and nutrient processes in streams. In contrast, dynamic transport models are more rigorous but not overly complicated, and offer substantially more flexibility and realism for assessing stream nutrient dynamics. However, it is yet unclear whether both approaches produce similar estimates of common uptake metrics. Thus, our objective was to compare the effectiveness of steady state approaches based on continuous nutrient enrichment to dynamic approaches based on short term enrichment for quantifying phosphorus (P) uptake dynamics using the common metric of uptake velocity (vf) in eight Wisconsin streams.
Results/Conclusions
The comparison of dynamically modeled P uptake metrics to steady state metrics yielded insights not readily apparent from steady state metrics alone. First, we observed contrasts in P uptake process rates over the time course of the continuous nutrient enrichment; uptake was significantly higher (p = 0.02) on the rising limb of the nutrient addition (mean vf = 1.7) than on the falling limb (mean vf=0.4). Indeed, P was often released during the falling limb, presumably due in part to physical sorption. Second, although uptake estimates were significantly higher in short-term enrichments (mean vf =2.1 vs. 1.2, p=0.01), broad patterns across systems were consistent with the two approaches. Recognizing that uptake lengths are generally long for streams in this region, and therefore predisposed to higher uncertainty that could have masked patterns, these results provide reassuring evidence that alternative nutrient enrichment strategies may facilitate the quantification and interpretation of biogeochemical processing rates in challenging situations, including flowpaths with long residence times (e.g. wetlands), large rivers, and changing conditions of flow and background chemistry.
